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Tuesday, December 7, 2010

Get The Most Out Of Variable Aperture Lenses


Often dismissed by “serious” photographers, these lenses offer some significant advantages

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This Article Features Photo Zoom

Sigma APO 70-200mm ƒ/2.8 EX DG Macro HSM

Tamron SP AF70-200mm F/2.8 Di LD (IF) Macro

AF-S NIKKOR 70-200mm ƒ/2.8G ED VR II
There are a number of zoom-lens “formulas,” but basically a telezoom changes the magnification (focal length) by moving elements in front of the aperture diaphragm.

A photographic lens’ ƒ-number is the ratio between its focal length and the diameter of the entrance pupil (the apparent size of the aperture as viewed from the front of the lens, not the physical diameter of the diaphragm opening). In a constant-aperture zoom (here, Canon’s EF 70-200mm ƒ/2.8L IS II USM), the front elements change the magnification of the image and the entrance pupil proportionately, so the ƒ-number remains constant throughout the focal-length range (the entrance pupil diameter must be 70/2.8, or 25mm, to provide ƒ/2.8 at an effective focal length of 70mm, and 200/2.8, or 71.4mm, to provide ƒ/2.8 at an effective focal length of 200mm). Note that in this lens, the actual aperture diameter is 25mm at all focal lengths; the optical magnification during zooming produces both the increased focal length and the increased entrance-pupil diameter.


Canon EF 70-200mm ƒ/2.8L IS II USM
How The Aperture Varies
Okay, we know that the effective aperture on a variable-aperture zoom lens varies, but exactly how does it vary? We wondered if a relatively inexpensive 70-300mm ƒ/4-5.6 zoom can provide you with a 200mm ƒ/4 in the midst of its range, along with the wide and tele ends. In other words, can you effectively get a 200mm ƒ/4 for the much lower price of a 70-300mm variable-aperture zoom?

We asked our friend and equipment expert Mark Comon of Paul’s Photo in Torrance, Calif., to check this for us, as he has access to a number of such zooms. Using the same testing procedure we describe in the “Testing Variable Apertures” sidebar, Comon evaluated several popular 70-300mm zooms, a 55-300mm, a 120-300mm and an 80-400mm, and the results are in Chart A. Considering that ƒ/4 is the maximum aperture at 70mm and 200mm is much closer to its maximum (slowest) focal length, we suspected you can’t get something for nothing and we were right, but the results still had some surprises. As you can see in the chart, the effective aperture rapidly narrows as you zoom to longer focal lengths, reaching ƒ/5.3 to ƒ/5.6 by 200mm with all eight lenses. If you need a 200mm ƒ/4, you’ll have to spring for an actual 200mm ƒ/4 prime lens. But you also can see that the lenses don’t follow a linear path across their variable-aperture range. Some go to a slower aperture relatively quickly and some stay at the faster aperture for more of the zoom range. This information is particularly useful as you consider your shooting priorities.


Sigma 70-200mm ƒ/2.8 EX DG OS HSM
Bottom Line
So will a low-priced variable-aperture zoom do the job for you? It depends on what you usually photograph. For example, if you’re a dedicated wildlife photographer, the biggest challenge for a wildlife lens is birds in flight. In our local group of bird photographers, we have several talented individuals who use stabilized 70-300mm variable-aperture zooms, and they have produced many sharp bird-in-flight shots right there with those produced by users of far more costly lenses. Having used both, I can say that the pro lenses autofocus noticeably more quickly, and the number of “keepers” is higher. But the budget-priced, variable-aperture zooms mentioned here can do the job, and very well. (Note that the lowest-priced 70-300mm zooms—those costing under $250, in general—are less suitable for wildlife action photography due to slower AF performance.) In situations where lightning-fast AF is less of an issue, the variable-aperture models are very well suited. Their generally lighter weight and reduced bulk make them excellent for hiking and traveling and as general-use zooms.

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